Metal components are commonly heat treated to establish their metallurgical and mechanical properties. Heat treatment parameters which influence the properties obtained include the rate at which the component is heated and then cooled, and the time and temperature at which the component is held.
The configuration of the component exerts an influence on the ability to achieve the desired heat treated properties. Components having a relatively simple geometry and sections of constant thickness are, in general, considerably easier to heat treat than are complex geometry components with widely varying section thicknesses. For example, in complex components, even though an overall rate of temperature change is imposed upon the component (e.g., the rate at which the temperature of the heat treatment furnace is raised), thin component sections heat up at a faster rate than more massive, thick component sections. This phenomenon also occurs when the component is cooled: thin sections cool faster than thick sections. Due to the different heating (or cooling) rates which the various sections of a complex component experience during heat treatment, the properties of thin sections may differ from the properties of thick sections. Also, if the heating (or cooling) rates of thick and thin sections are significantly different, the component may distort, or even crack, as the component is heat treated.
Integrally bladed rotors (IBR) are an example of complex metal components which are used in some gas turbine engines. See, e.g., commonly assigned U.S. Pat. No. 4,479,293 to Miller et al, and the patents referenced therein. One advantage provided by IBR rotor designs over conventional blade and disk designs is that an IBR is a one piece component, as opposed to the conventional assembly of a plurality of individual blades in a disk. The one piece IBR is therefore less prone to vibration damage and some of the other known disadvantages of conventional rotors.
In some IBRs the hub section is substantially more massive than are the blades which extend radially outwardly from the hub. Differences in section thickness of two orders of magnitude between the hub and blades are not uncommon. Accordingly, the heat treatment of such complex geometry components is difficult, for the reasons discussed above. In particular, it is difficult to achieve the heating and cooling rates required of the thick hub section without causing distortion and/or cracking of the thin blades.
The prior art shows several techniques for controlling the cooling rate during the heat treatment of metal components. See, for example, U.S. Pat. Nos. 3,558,367 to Eck and 3,703,093 to Komatsu et al. However, these techniques are useful only with components having relatively simple designs. Accordingly, improved techniques are needed, especially for complex geometry components like IBRs, which have nonuniform section thicknesses.